KR101401823B1 - Method and apparatus for calculating a laser shot file for use in an excimer laser - Google Patents

Abstract

A method and apparatus for calculating a laser shot file for use in an excimer laser, the method comprising: providing information about a desired ablation profile; Calculating a shot density of the desired ablation profile; And using a cost function to provide laser shots of the excimer laser on grid positions, wherein the threshold is determined based on the calculated shot density of the desired ablation profile.

Excimer laser, laser shot profile, grid width, ablation profile, calibration

Description

METHOD AND APPARATUS FOR CALCULATING A LASER SHOT FILE FOR USE IN EXCIMER LASER FIELD OF THE INVENTION [0001]

The present invention relates to a method and apparatus for calculating a laser shot file for use in an excimer laser, in particular using a dithering algorithm. The present invention is particularly suited for applying a laser shot file when producing a contact lens or an eyepiece (IOL) customized by laser ablation or laser ablation of the eye.

U.S. Patent No. 6,090,100 relates to an excimer laser system for vision correction with reduced thermal effects. Specifically, this patent relates to an apparatus and method for controlling an excimer laser system for removing tissue from the eye to perform various types of calibrations, such as near-sight, native, and astigmatism correction. In one disclosed embodiment, an excimer laser system provides a relatively large spot size that provides a relatively large coverage area of treatment per shot. While utilizing such a large spot size, shots are generally not "adjacent" to each other, but instead are superimposed to perform the desired degree of ablation at a particular point. To calculate the result of overlapping shots, one algorithm is used. In one method of calculating treatment patterns using large and constant spot sizes distributed throughout the treatment area, a dithering algorithm is used. Rectangular dithering, circular dithering and line-by-line oriented dithering are particularly referred to. Using any of a variety of shot dithering methods, an array of shots is generated for a constant spot size distributed throughout the treatment area for calibrating to the desired ablation. For each array, a grid having a constant grid width between the individual grid positions is used. In known dithering methods, the shape of the desired ablation profile, which is usually a continuous profile, must be transformed into a whole-numbered discrete density distribution. Here, the continuous profile represents the planned ablation, and the integer discrete density distribution represents a series of ablating flying spot laser pulses. The residual structure, i.e. the difference between the planned and achieved profiles, should be minimized. Correct solutions can be found primarily numerically, not at the right time. Thus, for this purpose, dithering algorithms are used. The profile is dispersed on a given grid. The algorithm uses a cost function or an advantage function to determine whether to provide a shot for each position of the grid. For this determination, only a few neighboring locations of the grid are typically considered. This dithering algorithm saves computation time without having to consider the actual size of the spot. Knowing the shot volume to be ablated by one laser shot is sufficient. However, under certain conditions, known dithering algorithms generate artifacts in portions of the profile, e.g., in low density regions where the next neighbor shot is too far away. Artifacts can also be generated in high density areas where shots are provided at almost any position. Also, positions without shots have too large gaps under the assumption that only a small number of neighboring positions are required.

With regard to the general background of dithering algorithms, reference is made to U.S. Patent No. 6,271,936, which is related to the field of digital image therapy. Specifically, the patent relates to a method for digitally monitoring continuous tone images using error diffusion, dithering, and over-modulation methods. Artifacts such as worms formed when black or white output pixels line up together in a region to be uniform are mentioned. This U.S. patent relates to a completely different technical field in that it provides a detailed description of these known methods. Among other differences, known laser printer systems use each constant resolution given as dots per inch, i.e., more dots per inch leads to better resolution. Moreover, known laser printers do not have the problem of dots overlapping and tangential, since they do not cause additional blackening when a point is frequently hit more than once. Rather, such a local area can be created by providing a corresponding number of dots in a given local area of an image having a predetermined gray level, to produce an image.

SUMMARY OF THE INVENTION [

It is an object of the present invention to provide an apparatus and method for calculating a shot of a shot for use in a refracted excimer laser wherein the difference between the projected profile and the achieved profile is minimized. This object is solved by the features of the claims.

For example, the desired ablation profile for correcting myopia has a maximum shot density at the center of the treatment area, while a minimum shot density along the peripheral boundary of the treatment area exists. Thus, the number of laser shots applied to the center of the treatment area is greater, especially in other subareas along the border of the treatment area.

For example, for primitive calibration, there is a minimum shot density at the center of the treatment area. On the other hand, ablation profiles require a greater number of laser shots along the perimeter boundaries of the treatment area.

The present invention is generally applicable to any ablation profile in which subregions having different shot densities are examined to determine any subregions with maximum shot density and / or any subregions with minimum shot density.

The overall concept of the present invention is based on the idea of adapting the dithering algorithm used to provide laser shots of an excimer laser when discretizing a given ablation profile on a given grid. The dithering algorithm uses a cost function to determine whether to provide shots for each position of the grid. Specifically, the shot density for obtaining a desired ablation profile is calculated first. Depending on the calculated shot density of the desired ablation profile, the dithering algorithm is adapted using the dynamic threshold used in the cost function for shot computation.

According to a preferred embodiment of the present invention, the threshold is selected from two or more different thresholds according to the minimum shot density and / or the maximum shot density of the desired ablation profile. Generally, a lower threshold is used for the desired ablation profile having a low shot density. A higher threshold is used for the desired ablation profile having a high shot density.

According to a preferred embodiment of the present invention, the first threshold is within a range of 0% to 20% of the maximum shot density of the ablation profile. Alternatively or additionally, the second threshold is a value in the range of 20% to 80% of the maximum shot density. Alternatively or additionally, the third threshold is a value in the range of 80% to 100% of the maximum shot density.

According to a more preferred embodiment of the invention, more than three thresholds are used, more preferably the threshold "TV (x, y)" is associated with the shot density "D (x, y)" .

More preferably, there is a linear relationship between the thresholds "TV (x, y)" and "D (x, y)"

Here, "a" is a positive factor in the range of O < a &lt; 1.5, and "x" and "y" are coordinates of the grid position at which calculation is performed.

Preferably, the threshold is set for each grid position according to a density function. More preferably, the threshold is set to a value equal to or similar to the value of the density function at each grid location.

The threshold value is preferably at least in the range of 80% to 110%, more preferably 90% to 100% of the value of the density function at each grid position. Therefore, the factor "a" of the formula 1 is preferably in the range of 0.8 to 1.1, more preferably 0.9 to 1.0. The best results can be obtained at a = 1.

The local shot density D (x, y) in the subarea around the grid position P (x, y) can be calculated by using the following formula, using the resection volume V _shot of the single laser _shot and the grid width G, Is calculated from the profile Z (x, y).

According to a preferred embodiment, a dithering algorithm is used to calculate the provision of laser shots of an excimer laser on grid positions. The dithering algorithm is adapted to the desired ablation profile by determining the optimized grid width of the grid to be used in the dithering algorithm. For a more detailed description of this aspect of optimizing the grid width, reference is made to our co-pending patent application entitled " Method and Apparatus for a Laser Shot File for a Refractive Excimer Laser. &Quot;

According to a preferred embodiment, the maximum value Z _max (x, y) of the profile and the grid width for the desired maximum density D _max (x, y) are obtained using the following equations.

In Equation 3, the minimum surrounding local shot density of the desired profile is calculated using a given grid width. Preferably, the grid width is calculated using Equation (4). The influence of the dynamic threshold is explained using two examples. As a first example, a treatment using a treatment area of about 5.5 mm is selected for a desired calibration of +4 dpt. This primitive correction is maximal resection along the annular portion around the center. The desired depth is about 26 μm. Approximately 445 laser shots are required to reach the result using a conventional excimer therapy laser. A grid width of 98 μm is selected to obtain a shot density along the annular portion of about 18%. In this example, ablation is calculated using a constant threshold. In the second ablation example, the treatment area is again 5.5 mm and the calibration is +4 dpt. The desired maximum depth is about 26 μm, and about 445 laser shots are required. For the second example, a dynamic threshold is used. The second example shows the benefit of using dynamic thresholds when calculating ablation.

According to a more preferred embodiment, the desired ablation profile is divided into two or more ablation sub-profiles. Then, for each ablation sub-profile, the respective shot density is calculated and the respective grid widths are determined based on the density of each calculated ablation sub-profile. Each sub-profile is calculated using dynamic thresholds. Thus, for a desired ablation profile, in which the contrast is very high, i.e. the difference between the maximum shot density and the minimum shot density is very large, the calculation of the laser shot file is preferably performed for each ablation sub- Is done in more than one run using different grid constants or grid widths. Thereafter, two or more laser shot files may be combined into one single laser shot file.

According to the invention, the calculated and provided laser shots are treated in an additional step of sorting to obtain a laser shot sequence. The classification is performed considering that any thermal effects should be avoided, i.e., that two consecutive laser shots are preferably provided at different grid locations spaced apart from each other within the treatment area.

The invention is further illustrated by means of examples with reference to the drawings.

1A is a diagram illustrating the location of laser shots for a first test using a constant threshold.

1B is a view showing a planned profile and an achieved profile as a section along the horizontal axis of FIG. 1A.

Figure 1C is a view showing the projected profile and the achieved profile as a section along the vertical axis of Figure 1A.

2A is a diagram illustrating locations of laser shots for a second test using a dynamic threshold in accordance with a preferred embodiment of the present invention.

Fig. 2B is a view showing the planned profile and the achieved profile as a section along the horizontal axis of Fig. 2A.

Figure 2C is a diagram showing the projected profile and the achieved profile as a section along the vertical axis of Figure 2A.

3 is a flow chart for calculating laser pulse patterns using a dithering algorithm.

4 is an illustration of an example of a sub-grid with weighting factors that can be used to weight neighboring error values.

Figures 1A, 1B and 1C illustrate the use of a conventional refractory excimer laser in a treatment region having a diameter of 5.5 mm and refraction to correct the source to a value of about +4 diopters using a laser shot having a diameter of 1 mm Lt; / RTI &gt; shows a simulated calculation of a laser shot file for use in an excimer laser. In this simulated first test, the grid width is 98 [mu] m. Thus, the distance between two neighboring grid points is 98 [mu] m. In this example, the grid points are arranged in rows and columns. A total of 445 laser shots are used to achieve ablation. Depending on the ablation volume of a single shot, the resulting treatment is expected to have a refraction of about +4 diopters as described above. 1A shows the center position of each of the 445 laser shots associated with one of the grid positions respectively marked with a "+" sign. At the upper right corner of Figure 1A, the grid is schematically shown as having a grid width of 98 [mu] m. Each of the illustrated laser shot center positions is arranged on one grid point of this grid. The diagram of Figure 1B shows the desired ablation profile, i.e. the ablation depth in microns for each X position, in dotted lines. The ablation depth is approximately 26 [mu] m at the annular portion of the treatment region at approximately x-2 and +2 at the x-position, and less at the center and both sides. The depth of ablation at the center is almost zero. This figure further shows the simulated result ablation profile as a solid line like the section taken along the horizontal axis through point 0-0 in Figure 1A. Similarly, Figure 1C further illustrates the desired ablation profile as a dotted line taken as a section along the vertical axis through point 0-0 in Figure 1A. Figure 1C further shows the resulting ablation profile as a solid line taken as a section along the vertical axis through point 0-0 in Figure IA. In Figure 1, the average shot density in the treatment area having a diameter of 5.5 mm in this example is about 18% (Figure 1A). The center position of each of the laser shots is located within a range of ± 2.7 mm in the X direction and ± 2.7 mm in the Y direction.

Figures 2A, 2B, and 2C show the results of a second test similar to those in Figures 1A, 1B, and 1C except that the dynamic threshold is used. Specifically, in this test, the shot density D (x, y) was used as the threshold TV (x, y). Therefore, the factor "a" is selected as a = 1 in Equation 2 above.

The use of certain thresholds for the first test results in artifacts such as a linewise arrangement of laser shot positions within the lower portion of ablation (Figure 1A). For example, as shown, several laser shots are provided at grid positions arranged along a horizontal sub-line at closer spacing. More laser shots are provided at grid locations arranged at larger intervals from this horizontal sub-line. Thus, the laser shots are not provided in the same manner, and thus deviations from the desired ablation profile occur (see FIG. 1C).

Comparison of the figures for the first test and the second test shows that the resulting ablation profile in the second test is better, i.e. the curve of the resulting ablation profile better follows the curve of the desired ablation profile 2B and 2C). Specifically, FIG. 1C shows that the resulting ablation profile deviates from the desired ablation profile, i. E. There is a shift relative to the right portion of the desired ablation profile. The dithering algorithm generates artifacts in portions of the resulting ablation profile that may depend on the order of computation of laser shots for each grid position. Shots are shifted in areas with a slope of the shot density. The shift depends on the depth of the desired ablation. In addition, artifacts such as worms can occur.

Because of the dithering algorithm, the input parameters are the shot volume of the laser shot and the desired ablation profile. Since the dithering algorithm operates irrespective of the beam diameter, it is not necessary to consider the beam diameter. The dithering algorithm provides the laser shot file as output. More specifically, a dithering algorithm is used for providing laser shots of the excimer laser on grid positions. Preferably, a cost function is used to determine whether a laser shot is provided for each grid position. Here, a determination is preferably made as to whether one or more laser shots are provided at a neighboring grid position (s) of a given grid position. Preferably, a dithering algorithm is used as disclosed in U.S. Patent No. 6,090,100.

Hereinafter, a preferred dithering algorithm will be described with reference to FIG. 3, which shows a flow chart illustrating an example of error diffusion. This dithering algorithm is based on the concept of error diffusion. Before the step of error diffusion, the desired ablation profile is calculated, for example based on the desired correction of the patient's eye or modification of the contact lenses or IOLs. This profile is stored in a grid with a specific grid width. For example, these grids have a value of 256 X 256 and cover an area of 15 ² mm ² . Error diffusion can be initiated at one edge in the grid, followed by line by line.

In a first step Sl, abstraction profile and dynamic threshold are determined using Equation (1), and the active dithering position is set to a point in one of the edges of the grid. Optionally, the desired grid width is calculated. The active dithering position represents the actual position in the grid being treated.

In the next step S2, a desired ablation value for the active dither position is obtained. In step S3, this desired ablation value is multiplied by the scaling factor f. The scaling factor f takes into account the laser pulses of different sizes and the arrangement width, i.e., the grid width. More specifically, the scaling factor is calculated as follows to obtain the desired shot density at this position (see equation (3)).

15 ² for the above-mentioned grid having 256 x 256 values which covers an area of ² mm, the grid width is 15 mm / 256 = 58 μm. Thus, the area of the smallest square the laser beam can be transmitted around is (58 μm) ² . Thus, the number of pulses calculated is reduced to account for the overlap of the laser pulses.

In the next step S4, weighted neighboring errors are added to the desired scaled abstraction value for the active dithering position. These weighted neighboring errors are preferably weighted sums of errors of already treated neighboring grid points. An example will be described later.

In a further step S5, a determination is made whether the value obtained is greater than a predetermined threshold value. Thus, the value for each grid point and the sum of the weighted errors of adjacent grid points will be compared to the threshold. If the value is not larger than the threshold value T (x, y), the process proceeds to step S9. If the value is greater than the threshold value, the laser pulse is set for this grid position in step S6. One laser pulse is subtracted from the density value. Then, in step S7 it is determined whether the new value is still greater than the threshold. In step S8, if the new value is larger than the dynamic threshold value, it is determined that a shot overflow error has occurred. That is, if it is necessary to set more laser pulses at one grid position, the algorithm should be stopped with an error. By using the calculated grid width using Equation 4, such an error can be prevented. In this embodiment of error diffusion, a maximum of one laser pulse for each grid position is allowed.

On the other hand, in step S9, if the new value is not greater than the threshold value, this new value is stored as an error for this particular grid position. This will be used when treating neighboring locations for computation for additional dithering positions.

In the next step S10, a determination is made as to whether the line has been completed, otherwise, the next point in the same line is selected as the active position in step S11, and the above-described treatment is repeated. If the line is complete, a determination must be made as to whether a new line is present in step S12, and in such a case, the first point in the new line is selected as the active position in step S13, and the treatment is repeated. On the other hand, if there is no new line, the treatment ends in step S14. The grid point errors described above represent ablation errors made at a particular grid point. For each grid point treated, this error is the sum of the desired ablation value + weighted neighboring errors-laser pulse ablation depth (if a laser pulse is set for that position).

4 shows an example of weighting errors of neighboring grid points. More specifically, Figure 4 shows a sub-grid of 7 x 7 grid points, with the active dithering position shown at the center. In this case, the weighting function is determined as (8 / distance) for the distance measured in grid point units. The sum of the errors is then normalized by dividing by the sum of all weight factors used, which is 70.736. As can be seen from FIG. 4, the white locations represent untreated grid locations. Thus, before determining whether a laser pulse should be set at a given grid position, the error induced during the treatment of adjacent grid points must be added to the theoretical abstraction value for that grid point. Errors in neighboring grid points are not simply added, but are weighted by the distance to their active grid point. Each of the weighting factors is shown in FIG. It should be noted that this is but one possible way to sum the peripheral errors and works well.

It should be noted that the above-described dithering algorithm is only an example for using the present invention.

The laser shot sequence can then be determined using an individual classification algorithm. The classification may be performed to prevent thermal effects. Thus, any two successive laser shots should preferably be provided at two grid positions spaced apart from one another. Preferably, for every four shots, a laser shot is provided at the same position as the first shot.

The above disclosure and description of the present invention are for the purpose of illustration and description, and modifications of construction and operating methods may be made without departing from the scope of the present invention.

Claims (36)

CLAIMS What is claimed is: 1. A method of calculating a laser shot file for use in an excimer laser for performing refractive laser treatment of the eye or for manufacturing customized contact lenses or eyepieces, Providing information about a desired ablation profile; And Steps using a dithering algorithm Lt; / RTI &gt; Wherein the dithering algorithm is adapted to the desired ablation profile by using a dynamic threshold to calculate the laser shot file. The method according to claim 1, Using the dithering algorithm to discretize the desired ablation profile on a given grid; And Determining whether to provide a laser shot of the excimer laser on the grid position for each grid position &Lt; / RTI &gt; 3. The method of claim 2, wherein the dithering algorithm utilizes a cost function to determine whether to provide a laser shot of the excimer laser on the grid position for each grid position. 4. The method of any one of claims 1 to 3, further comprising calculating a shot density to obtain the desired ablation profile, wherein the dynamic threshold is defined according to the calculated shot density of the desired ablation profile Way. 4. The method of any one of claims 1 to 3, wherein two or more different thresholds are used according to the desired ablation profile. 6. The method of claim 5, wherein a first threshold is used for a desired ablation profile having a low shot density, and / or a second threshold is used for a desired ablation profile having a high shot density, Lower than the threshold. 7. The method of claim 6, wherein the first threshold is within a range of 0% to 20% of the maximum shot density of the desired ablation profile, and / or the second threshold is within a range of 20% to 80% And / or the third threshold is a value within a range of 80% to 100% of the maximum shot density. 4. The method according to any one of claims 1 to 3, wherein the threshold TV (x, y) is related to the shot density D (x, y) of the desired ablation profile according to the following equation: &Quot; (1) &quot;

4. The method according to any one of claims 1 to 3, wherein the threshold TV (x, y) is linearly related to the shot density D (x, y) of the desired ablation profile according to the following equation: &Quot; (2) &quot;

Here, "a" is a factor within the range of O < a &lt; 10. The method of claim 9, wherein the threshold is set to a value equal to or similar to the value of the shot density. 4. The method according to any one of claims 1 to 3, wherein the grid width of a given grid is determined based on the calculated shot density of the desired ablation profile. 4. The method of claim 2 or claim 3, wherein determining whether to provide a shot on a given grid position is such that a corresponding determination of neighboring grid positions of the given grid position is considered. 4. The method according to any one of claims 1 to 3, Dividing the desired ablation profile into two or more ablation sub-profiles; Calculating a shot density of each of the ablation sub-profiles; And Determining respective grid widths based on each calculated shot density of each of the ablation sub-profiles; &Lt; / RTI &gt; 4. The method of claim 2 or 3, further comprising classifying the calculated and provided laser shots. 4. The method according to any one of claims 1 to 3, wherein the excimer laser is a laser beam having a constant spot size with a diameter of 0.5 mm to 3.5 mm. An apparatus for calculating a laser shot file for use in an excimer laser for performing refractive laser treatment of the eye or for manufacturing customized contact lenses or eyepieces, Means for providing information about a desired ablation profile; And Means for using a dithering algorithm / RTI &gt; Wherein the dithering algorithm is adapted to the desired ablation profile by using a dynamic threshold to calculate the laser shot file. 17. The method of claim 16, When using the dithering algorithm, Means for discretizing said desired ablation profile on a given grid; And Means for determining whether to provide a laser shot of the excimer laser on the grid position for each grid position Lt; / RTI &gt; 18. The apparatus of claim 17, wherein the dithering algorithm utilizes a cost function to determine whether to provide a laser shot of the excimer laser on the grid position for each grid position. 19. The method of any one of claims 16-18, further comprising: means for calculating a shot density to obtain the desired ablation profile, wherein the dynamic threshold is defined according to the calculated shot density of the desired ablation profile Device. 19. The apparatus of any one of claims 16-18, further comprising means for selecting two or more different thresholds according to the desired ablation profile. 21. The method of claim 20, wherein a first threshold is selected for a desired ablation profile having a low shot density, and a second threshold is selected for a desired ablation profile having a high shot density, 2 Lower than threshold. 22. The method of claim 21, wherein the first threshold is within a range of 0% to 20% of the maximum shot density of the desired ablation profile, and / or the second threshold is within a range of 20% to 80% And / or the third threshold is a value within a range of 80% to 100% of the maximum shot density. 19. The apparatus of any of claims 16-18, further comprising means for determining a threshold TV (x, y) associated with the shot density D (x, y) of the desired ablation profile according to Equation . &Quot; (1) &quot;

19. The apparatus according to any one of claims 16 to 18, further comprising means for determining a threshold TV (x, y) as a linear relationship with the shot density D (x, y) of the desired ablation profile according to the following equation Comprising a device. &Quot; (2) &quot;

Here, "a" is a factor within the range of O < a &lt; 25. The apparatus of claim 24, further comprising means for setting the threshold to a value equal to or similar to the value of the shot density. 19. Apparatus according to any one of claims 16-18, further comprising means for determining a grid width of a given grid based on a calculated shot density of the desired ablation profile. 19. The apparatus of claim 17 or 18, wherein the means for determining whether to provide a shot on a given grid position utilizes information on a corresponding determination of neighboring grid positions of the given grid position. 19. The method according to any one of claims 16 to 18, Means for dividing a desired ablation profile into two or more ablation sub-profiles; Means for calculating a shot density of each of the ablation sub-profiles; And Means for determining a respective grid width based on a calculated shot density of each of said abstraction sub- Lt; / RTI &gt; 19. The apparatus of claim 17 or 18, further comprising means for classifying the calculated and provided laser shots. 19. The apparatus of any one of claims 16 to 18, wherein the excimer laser provides a laser beam having a constant spot size of 0.5 mm to 3.5 mm in diameter. 10. The method of claim 9, wherein "a" is a factor within the range of 0.8 &lt; 32. The method of claim 31, wherein a = 1. 16. The method of claim 15, wherein the excimer laser provides a laser beam having a constant spot size of 1.0 to 2.0 mm in diameter. 25. The apparatus of claim 24, wherein "a" is a factor within the range of 0.8 &lt; 35. The apparatus of claim 34, wherein a = 1. 31. The apparatus of claim 30, wherein the excimer laser provides a laser beam of a constant spot size having a diameter between 1.0 and 2.0 mm.

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